The Fluke 845 does not use a mercury cell.
In a lot of these type systems, the mercury cell was used in a ratiometric configuration. Its exact voltage is not important. What is important is a stable voltage over your measurement interval. Lithium cells are usually OK.
Also the NE-2 neon lamps are still made, readily available, and dirt cheap.
Standard chicken soup for a twitchy 845 is to clean all the switch contacts, check the electrolytics, check the chopper system.
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